1. Field of the Invention
The present invention relates to a drain valve for use in steam heating, etc.
2. Description of the Prior Art
In a diesel locomotive running in a cold region, for example, steam heating is generally adopted from structural and economic points of view. A number of drain valves for discharging steam condensate are installed at suitable positions along a steam flow tube at regular intervals. A prior art drain valve of this type is accommodated within a hollow valve casing communicating with the aforementioned steam flow tube and comprises a drain valve head for opening or closing a discharge port formed in the lower portion of the valve casing, a guide cylinder for supporting a drain valve stem so that it is capable of ascending or descending, a large-sized valve body which retains the guide cylinder on the upper portion thereof and has a plurality of vents formed in the side portion thereof, and stainless steel bellows attached between the drain valve head and the guide cylinder within the valve body. The bellows, change and cause the drain valve head to descend to automatically stop up the discharge port when the interior of the valve casing is brought to a high-temperature state with steam and, to the contrary, when part of the steam becomes cold and converts into condensate and the condensate flows into the valve body to bring the interior of the valve body to a low-temperature state, the bellows contract and cause the drain valve head to ascend to automatically open the discharge port, with the result that the condensate is spontaneously discharged out of the valve body through the discharge port.
However, since the bellows which cause the drain valve head to ascend or descend by utilization of a temperature difference has a construction such that it has gas sealed therein and undergoes stretching and contracting, there is a fair possibility of the gas escaping from the bellows to considerably lower the stretching and contracting functions of the bellows. This brings about disadvantages in that the opening or closing of the discharge port is impeded, that an auxiliary valve is additionally required to install for the purpose of supplementing the impediment, and that the durability is inferior due to its own structure. Further, since the bellows per se is very expensive, it is confirmed that use of the bellows poses a problem from an aspect of the manufacturing cost conjointly with the inferior durability of the bellows. Furthermore, the valve body is required to have a size large enough to accommodate therein the drain valve head, guide cylinder, and bellows as described above and, therefore, the inside structure is made complicated.